1. Field of the Invention
The present invention relates to a composite antenna for use in an on-vehicle mobile radio device or the like.
2. Background Art
A monopole antenna that has a rod shape and one operating frequency has been recently and generally used as an antenna for use in an on-vehicle mobile device or the like. Wire length (antenna length) of the monopole antenna, however, must be about ¼ wavelength or longer of the operating frequency. The monopole antenna is not suitable for use in a mobile body such as an automobile or a portable apparatus.
Service such as a car telephone, a global positioning system (GPS), a vehicle information communication system (VICS), or data communication is diversified in a mobile body such as an automobile. When a monopole antenna is loaded into the mobile body in each application, the number of installed antennas must be increased. These antennas are generally attached to the outside of the car body, and attaching many antennas damages the appearance of the automobile. Therefore, a small composite antenna capable of adapting to a plurality of operating frequencies required for a plurality of applications is proposed. This composite antenna is attached to the inside or outside of the car body.
FIG. 6 is a longitudinal sectional view of a composite antenna disclosed in Japanese Patent Unexamined Publication No. 2000-183643. Composite antenna 700 is formed by integrating first antenna 500 and plane antenna 600.
First antenna 500 has conductor ground plate 510, antenna element 530, feeding terminal 520, and short circuit terminal 540. Conductor ground plate 510 is formed of a conductive plate. Antenna element 530 is formed of a conductive plate smaller than conductor ground plate 510, is separated from conductor ground plate 510 by predetermined set interval 550, and is disposed in parallel with conductor ground plate 510. One end of feeding terminal 520 is electrically connected to the center of antenna element 530 at feeding point 521, and the other end thereof is penetrated through the center of conductor ground plate 510 in an insulating state and connected to signal source 400 on the back side of conductor ground plate 510. Short circuit terminal 540 is formed of a conductive material, and electrically connects antenna element 530 to conductor ground plate 510 at a position separated from the center of conductor ground plate 510 by predetermined distance 650.
Plane antenna 600 includes planar antenna element 630 and feeding terminal 620. Antenna element 630 is placed on the upper surface of antenna element 530. One end of feeding terminal 620 is electrically connected to antenna element 630 at feeding point 621. The other end thereof is penetrated through conductor ground plate 510 and short circuit terminal 540 in an insulating state, and is connected to signal source 450 on the back side of conductor ground plate 510.
In this configuration, composite antenna 700 transmits or receives two frequencies, namely operating frequencies fa and fb, with first antenna 500 and plane antenna 600, respectively.
For making the conventional composite antenna adapt to three or more operating frequencies, however, the entire height of the composite antenna must be increased. In other words, for adapting to three or more operating frequencies, a second antenna similar to first antenna 500 must be formed on first antenna 500, and plane antenna 600 must be placed on the upper surface of the second antenna. Therefore, disadvantageously, the larger the number of operating frequencies, the bulkier the composite antenna is.
Japanese Patent Unexamined Publication No. H08-237025 discloses a composite plane antenna that is apparently one antenna and can support many kinds of communication services. The composite plane antenna has the following elements:                a folded monopole antenna with a metal plate that is formed by short-circuiting a conductor ground plate and a metal loaded plate with a short-circuiting means;        one or more antennas supported over the loaded plate; and        a feeding means that penetrates the folded monopole antenna and feeds power to one or more antennas.Also in this configuration, the height of the entire antenna must be disadvantageously increased for adapting to three or more operating frequencies.        
The present invention addresses the conventional problems, and provides a thin and small composite antenna capable of adapting to three or more operating frequencies.